Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC's with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in optical photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC's have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on optical lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate dramatically in sub-wavelength lithography. Another difficulty with sub-wavelength photolithography is that, as two mask patterns get closer together, diffraction problems occur. At some point, the normal diffraction of the exposure rays start touching, leaving the patterns unresolved in the resist. The blending of the two diffraction patterns into one results from all the rays being in the same phase. Phase is a term that relates to the relative positions of a wave's peaks and valleys. One way to prevent the diffraction patterns from affecting two adjacent mask patterns is to cover one of the openings with a transparent layer that shifts one of the sets of exposing rays out of phase, which in turn nulls the blending.
This is accomplished by using a special type of photomask known as a phase shift mask (PSM). A typical photomask, referred to as a binary mask, affects only one of the properties of light, the intensity. Where there is chromium, which is an opaque region, an intensity of zero percent results, whereas where the chromium has been removed, such that there is a clear or transparent region, an intensity of substantially 100 percent results. By comparison, a PSM not only changes the intensity of the light passing through, but its phase as well. By changing the phase of the light by 180 degrees in some areas, the PSM takes advantage of how the original light wave adds to the 180 degree wave to produce zero intensity as a result of destructive interference.
Types of PSM's include alternating PSM's and attenuated PSM's. The latter is of interest here, and is also referred to as a half-tone PSM. Unlike a binary photomask, an attenuated PSM has a dark region, referred to as the shifter region, or shifter film, that allows some light to transmit through. By comparison, the dark region of a binary photomask allows no light to transmit through. A typical attenuated PSM may have a shifter film that only allows less than ten percent of the light to pass through, such as only three-to-eight percent of the light to pass through. By comparison, a high-transmittance attenuated PSM may have a shifter film that allows a greater percentage of the light to transmit, such as ten-to-twenty-five percent, or more, such as forty-five percent.
FIGS. 1A and 1B show the comparison between a standard binary photomask and an attenuated PSM. The standard binary photomask 100 of FIG. 1A has a clear region 102, such as quartz, and an opaque region 104, such as chromium. When the photomask 100 is subjected to the light 106, the opaque region 104 blocks the light, so the only light that passes through the photomask 100 is that which the opaque region 104 does not block. By comparison, the attenuated PSM 120 of FIG. 1B also has a clear region 108, such as quartz, and a semi-opaque or semi-dark region 110, the latter which is a shifter film. The shifter film may be molybdenum silicide (MoSiOxNy, or Mo—Si), chromium fluoride (CrF), or zirconium-type materials, such as ZrSiO. When the attenuated PSM 120 is subjected to the light 112, the semi-opaque region 110 only blocks a portion of the light. Thus, the light completely passes through the clear region 108, and only anywhere from three percent or more—but less than one-hundred percent of—the light passes through the semi-opaque region 110.
A problem with high-transmittance attenuated PSM's, however, is that they are difficult to inspect for defects. This is because most inspection tools rely on light, where an operator examines the semi-opaque regions as compared to the clear regions to ensure that the regions have been accurately created. However, since both the semi-opaque regions and the clear regions of attenuated PSM's transmit light, and since the semi-opaque regions of high-transmittance attenuated PSM's may transmit as much as forty-five percent or more of the light, it is difficult for the operator to distinguish the semi-opaque regions from the clear regions. That is, there is low contrast between the semi-opaque regions and the clear regions of high-transmittance attenuated PSM's. A limited solution is to decrease the sensitivity of the inspection tools, to increase contrast. However, this means that critical defects of such high-transmittance attenuated PSM's may escape detection by the operator, leading to improperly fabricated semiconductor devices, and thus to wafer scrap.
Therefore, there is a need to overcome these disadvantages associated with inspection of high-transmittance attenuated PSM's. Specifically, there is a need for inspecting high-transmittance attenuated PSM's in a way that overcomes the inherent low contrast between the semi-opaque regions and the clear regions of such PSM's. Such inspection should be able to be performed without having to decrease the sensitivity of the inspection tools, so that critical defects do not escape detection. For these and other reasons, there is a need for the present invention.